Multiplex systems are, of course, commonly used in telephone systems for selecting and forming transmission channels between subscribers' lines and similar types of systems have been heretofore proposed for musical systems, using multiplexing for the generation of tones from the operation of keyboard switches. One such system is disclosed in the Watson U.S. Pat. No. 3,610,799, issued Oct. 5, 1971 in which a digital decoder and a switching array are used in conjunction with an encoder to develop a time division multiplex signal in the form of a serial output signal on a single conductor emanating from the encoder. The serial output signal so developed contains assignments of the notes which are associated with depressed keys. The Watson patent also discloses the use of a digital multiplex arrangement wherein information in serial digital form is used for the selection of generator circuits for generating tones by the use of digital techniques, the tones having wave shapes controlled from a time-shared read-only memory and register and calculator circuits. With such a system, there is a possiblity of eliminating or reducing the number of certain components and in reducing the complexity of wiring as used in prior systems, but from a practical standpoint, the implementation of the system to provide an instrument which is both operative and readily controlled involves the design and construction of complex digital circuits and would appear to be difficult and expensive.
The Rossum U.S. Pat. No. 3,986,423 issued Oct. 19, 1976 provides another prior art disclosure of a musical instrument or synthesizer in which multiplexing is employed. The Rossum patent discloses a system using digital multiplexing and having a plurality of channel circuits each of which includes a voice controlled oscillator controlled from a sample and hold circuit, the sample and hold circuits of all channels having inputs connected to the output of a digital-to-analog converter. Signals are applied to the digital-to-analog converter from the output of a read-only memory to which address signals are applied in synchronism with the scanning of a keyboard. Strobe signals are applied to the sample and hold circuits from comparison circuits in the respective channel circuits, each comparison circuit being operative to compare the signal applied from the address counter currently with that applied in a previous cycle and stored in a storage register. Channel selection logic is used to control the relationship between the channel circuits and actuated keys, by controlling the signals initially applied to the storage registers. The number of channels may be much less than the number of keys in the keyboard.
There are many other disclosures in the prior art relating to the use of multiplexing in musical instruments, the generation of desired wave shapes and the control of the envelope of a sound produced in response to each actuation of a key of a keyboard.
The White U.S. Pat. No. 3,006,228 issued Oct. 31, 1961 discloses a circuit for use in musical instruments in which the outputs of a series of frequency dividers are combined through gate circuits and switches to produce tones having desired tonal qualities.
The Cordry U.S. Pat. No. 3,297,812 issued Jan. 10, 1967 discloses an electric organ having single contact key switches and having function switches operable to determine the pitch of tones to be reproduced with a plurality of "And" gates equal in number to the number of key switches and to the number of function switches.
The Munch U.S. Pat. No. 3,417,188 issued Dec. 17, 1968 discloses a preference circuit for electronic musical instruments using pulse amplitude discrimination and a zero-crossing detector.
The Deutsch U.S. Pat. No. 3,515,792 issued June 2, 1970 discloses a digital organ in which wave shapes to be reproduced are stored in digital form in a memory in the form of a diode array which is scanned by means including ring counters to each of which a clock signal is applied at a frequency equal to a predetermined multiple (48) of the frequency of the tone to be reproduce. The Deutsch patent also discloses attack and decay control circuitry and also a frequency synthesizer in which notes such as C tones are produced from an oscillator through a series of cascaded divide-by-two circuits. B tones are produced in such similar fashion from a multivibrator controlled from a circuit which compares its output frequency divided by 185 with the output of the C tone oscillator divided by 196. B flat or A sharp is produced in a similar fashion from B natural, and so on.
The Arsem et al U.S. Pat. No. 3,696,201 issued Oct. 3, 1972 discloses a digital organ in which pulse producing means are controlled from key switches to produce repeating trains of pulses at repetition rates determined by the keys, such pulse trains being applied to a boxcar integrator to produce an audio tone.
The Deutsch U.S. Pat. No. 3,697,661 issued Oct. 10, 1972 discoses a multiplexed pitch generator system in which the function of mechanical intermanual couplers of conventional organs is performed electronically, using a plurality of shift registers and coincidence comparison circuitry.
The Reinagel U.S. Pat. No. 3,733,955 issued May 22, 1973 discloses a synthesizer keyboard arrangement wherein a single resistance string is used in independently controlling the frequencies of two voltage controlled oscillators at the same time.
The Colin U.S. Pat. No. 3,828,110 issued Aug. 6, 1974 discloses control circuitry for controlling a voltage controlled oscillator and an envelope generator from a keyboard and voltage divider circuit, a sample-and-hold circuit being provided.
The Nakajima U.S. Pat. No. 3,836,692 issued Sept. 17, 1974 discloses signal selecting system for an instrument having memory means and tone signal sources corresponding to one octave, frequency divider means coupled to the tone signal sources, preference circuit means and octave selecting means.
The Tomisawa U.S. Pat. No. 3,882,751 issued May 13, 1975 discloses an electronic musical instrument employing wave shape memories and also a frequency information memory and memories for control of the form of the envelopes of tones produced. The memories are controlled from a key assignor which includes a key address code memory which is capable of storing key address codes up to the same number as a maximum number of musical tones to be simultaneously reproduced, twelve channels being used in the illustrated embodiment. Shift registers are provided in the key assignor means and wave shape generating means and envelope wave shape generating means which have a number of stages corresponding to the number of channels and which are driven in synchronism.
The Morez et al U.S. Pat. No. 3,902,397 issued Sept. 2, 1975 discloses an electronic musical instrument including a couples control unit for shifting the position of pulses within a train of pulses which are encoded in time position in response to operation of keys and also for individually controlling the amplitude of pulses within the train. Sample and hold units are provided, one for each of the switches in one of a group of switches such as groups of upper manual, lower manual and pedal switches.
The Deutsch U.S. Pat. No. 3,929,053 issued Dec. 30, 1975 discloses a circuit arrangement for production of glide and portamento, wherein a generated tone has a frequency proportional to a number of digital form, such number being modified in increments through divider, adding and accumulator circuits.
The Southard U.S. Pat. No. 3,955,468 issued May 11, 1976 discloses an electronic musical instrument using digital multiplexed signals and having a plurality of keyboards with an arrangement for scanning the key switches in both keyboards simultaneously and for developing a corresponding digital signal.
The Sakashita U.S. Pat. No. 3,977,290 issued Aug. 31, 1976 discloses a circuit for generating a signal at the correct frequency to produce a musical tone, using a pulse counter having a variable frequency dividing ratio and a ratio-setting signal-generating circuit.
The Oya U.S. Pat. No. 3,981,217 issued Sept. 21, 1976 discloses a key assigner circuit in which instead of scanning key switches, changes in the conditions thereof are simultaneously and parallelly delivered therefrom.
The Obayashi et al U.S. Pat. No. 3,982,460 issued Sept. 28, 1976 discloses a circuit for producing a musical tone waveform in which the waveform to be reproduced is sampled at sampling points determined from the intersection with dividing lines at equal amplitude intervals, the time intervals between adjacent sampling points being thus variable. Information as to each sampling point is entered into a memory as a digital signal. Additional features relate to the development of digital signals corresponding to the increase, decrease or equality at each sampling point with respect to a preceding sampling point and to a provision of an envelope setting arrangement in which the envelope desired to be reproduced is subjected to sampling with the analog of each sampling point being set up as a digital signal.
The Kugisawa U.S. Pat. No. 3,982,461 issued Sept. 28, 1976 discloses an arrangement similar to that of the Obayashi U.S. Pat. No. 3,982,460 and is directed primarily to the generation of increase, decrease or equality signals from comparison of signals from sampling points to those from preceding sampling points and also to envelope setting means.